Field of the Invention
The invention relates to a backlight module for a liquid crystal display.
Description of the Related Art
Liquid crystal in a panel of LCDs utilizes a backlight module to provide a light source. The backlight source generally employs a light emitting diode (LED) or a cold cathode fluorescent source (CCFL) as the light source. However, the divergence angle of the light source is relatively large, and thus the energy utilization rate is generally less than 10%. Thus, the light emitted by the backlight module requires to be collimated.
FIG. 1 shows a backlight module capable of producing a collimated light. The backlight module includes: a light guide plate 10, a prism sheet 12, and a diffusion sheet 14. Light enters the light guide plate 10 via an incident plane 100. A part of the light directly emerges from the emergent plane 102, another part of the light emerges from a bottom plane 104, and the majority of the light is reflected one or several times inside the light guide plate 100 and is eventually ejected out of the light guide plate 10 via the emergent plane 102. The light ejected from the emergent plane 102 of the light guide plate 10 enters the prism sheet 12 via a prism plane 122, and a transmission direction of a majority of the light is changed by a plurality of prism structures disposed on the prism plane 122 and the light is then ejected in a direction of a normal line of a top plane 124, and travels to the diffusion sheet 14.
Although the backlight module is capable of providing collimated light, the divergence angle of the collimated light is too large and it is difficult to effectively decrease the angle. When the size of the reflection structure 128 is relatively large, the divergence angle of the reflected light is relatively large, and therefore, in order to decrease the divergence angle of the emergent light and to improve the collimation of the emergent light, the reflection structure 128 must be precisely manufactured, thereby increasing the difficulty and the production cost of the manufacturing process.
FIG. 2 shows a compact collimating reflection sheet (CCR) capable of producing a parallel beam. The CCR is disposed at bottom of the backlight module, and the whole structure has a ship-like shape. The light source is disposed at two ends of the collimation reflection sheet. The CCR includes an incident plane, which is also a reflective plane and has a zigzag structure. Each zigzag element has different angle and gradient. The light of different angle sent out by the light source is reflected by the zigzag structures having different angles and different gradients, and the transmission of the light is changed to eject the light perpendicularly to the liquid crystal screen.
Although the structure depicted in FIG. 2 is capable of producing a collimated beam, the light emitted from the light source is continuous and has a large angle and the reflection sheet of the collimated beam is in a continuous zigzag structure. Thus, the collimation effect of the produced collimated beam is not ideal. Meanwhile, the discontinuous zigzag structure is finely designed, and each zigzag has a specific angle and gradient, so that the processing difficulty is very large and the production cost is relatively high.